Smoothness always use alpha diffuse when using the constant source

Steps to reproduce:

1. Create a terrain
2. Create a terrain layer with only a diffuse map with an alpha channel.
3. Paint it.
4. Change the Smoothness source in the Channel Default Values from "diffuse alpha channel" to "Constant"

Actual results: The smoothness is still affected by the diffuse alpha channel.

Expected results: The smoothness should be a constant value, so totally uniform.

Reproducible with versions: 6.3

Not reproducible with versions: 

Can’t test with versions: 

Tested on (OS): Windows

Notes:

• It is a regression but I don't know the version where the feature was broken.